The detection of CO (carbon monoxide) in air is extremely relevant to a range of applications in buildings. On the one hand CO is odorless, but at the same time highly toxic. Accordingly, the maximum permissible concentration of CO at the workplace is expressed by an OEL value (OEL=occupational exposure limit) of currently only 30 ppm. If firing plants or flues are thus present in an interior space, conditions of inadequate combustion, i.e. due to lack of air, can result in carbon monoxide occurring in a concentration of several vol. % in the flue gas. If there is a leak in the flue tube or if flue exhaust air escapes into the interior space due to unfavorable pressure conditions—for example as a result of a poorly drawing flue or due to the operation of a fume extraction hood in the vicinity—, toxic CO concentrations can build up. In unfavorable circumstances these can be sufficient to cause damage to health, even leading to death due to poisoning. Thus, for example, in its “Carbon Monoxide Poisoning: Fact Sheet” the Center for Disease Control and Prevention quotes a number of 500 fatalities annually due to CO poisoning for the USA.
Another field of interest with regard to the detection of CO results from the fact that massive amounts of CO are formed in smoldering fires. Apart from the risk to people from poisoning, therefore, the gas can also be used as an indicator of combustion and consequently its detection can also be utilized for early detection of fire situations.
There is therefore a high demand for CO monitoring devices which continuously monitor the ambient air for the occurrence of CO. It is important with regard to the usability of devices of this kind that they provide a reliable warning without giving rise to false alarms. This means that a CO detector must be able to detect the carbon monoxide even when certain other gases are also present in the environment. Such gases are typically odorous gases, for example volatile organic hydrocarbons (VOC), alcohols which escape into the air when alcohol is consumed or from cleaning agents, relative air humidity and oxidizing gases (NO2, O3) infiltrating from the outside air. Fluctuating air temperatures must also not trigger any false alarms.
A known structure for detecting gases is a GasFET, i.e. a field-effect transistor structure embodied for detecting gas. At its gate the GasFET has a particularly gas-sensitive layer. Due to a gas-induced change in the electron work function an additional potential in the order of typically 10-100 mV, which acts as an additional gate voltage on the transistor and can be measured, results at the gas-sensitive layer.
DE 10 2004 019 638 A1 discloses a FET-based CO sensor which is based on a base material having little catalytic activity, for example gallium oxide, which is provided with a catalytically active noble metal dispersion (e.g. Pt or Pd) for the purpose of chemical activation. As a result of the catalytic activation the sensor becomes much more sensitive to CO. A FET-based CO sensor is also known from US 2007/0181426 A1. This sensor is based on a metal oxide as base material and an oxidation catalyst provided thereon. From the same publication it is also known to use the sensor for detecting alcohols or hydrogen.
The CO sensors are extremely cross-sensitive to solvents such as ethanol. Alcohol concentrations of up to several 100 ppm can occur when alcohol-based cleaning agents are used in a building as well as due to spilt alcohol from beverages. These produce the same signal level in the sensor as the CO concentration of 30 ppm that is required to be detected. Consequently incorrect measurements can easily be caused by ethanol.
It is known to provide active carbon filters upstream of a sensor element. The active carbon filters absorb ethanol and allow CO to pass. However, if ethanol is present for a protracted length of time the filter becomes saturated, whereupon it also allows ethanol to pass. Reliable filtering of ethanol is therefore not guaranteed even with active carbon filtering.